1. Field of the Invention
This invention relates to the filtration of a molten metal. More particularly, this invention relates to the filtration of a molten metal such as a molten aluminum alloy using cake mode filtration on a moving filter cloth.
2. Description of the Related Art
The production of aluminum base alloys has become of increasing interest due to the combination of low weight and high strength which such an alloy can be made to possess. However, the production of aluminum base alloys such as aluminum-magnesium or aluminum-lithium is significantly more difficult than that of other aluminum base alloys due to the reaction of these alloys with refractory linings in the furnace, the rapid rate of oxidation of lithium and magnesium, hydrogen pick-up by the molten alloy, caustic fume evolution, and composition gradients in the cast ingot due to the propensity of lithium and magnesium to oxidize during processing of the molten alloy.
After degassing the molten aluminum, any remaining solids or inclusions such as oxides must be removed by filtration. However, the use of conventional filtration media has been unsatisfactory due to formation of compressible cakes which rapidly plug a conventional static cake-mode or deep bed filter. Furthermore, the chemically aggressive nature of aluminum base alloys such as aluminum-lithium alloys quickly degrade conventional ceramic-based filter media.
In a paper coauthored by me entitled "Molten Metal Filtration: Fundamentals and Models", given at the Proceedings of the Technical Sessions sponsored by the TMS Light Metals Committee at the 113th Annual Meeting on Feb. 27-Mar. 1, 1984 and published by the Metallurgical Society of AIME in Light Metals 1984 at pp 1281-1304, the removal of inclusions from an aluminum melt using the separation processes of sedimentation, floatation, cake mode filtration, and deep bed filtration was discussed and an in-depth treatment of cake mode and deep bed filtration was given.
In another paper which I coauthored entitled "An Experimental Technique for Determining Specific Cake Resistance Values in the Cake Mode Filtration of Aluminum Alloys", given at the Proceedings of the Technical Sessions sponsored by the TMS Light Metals Committee at the 114th Annual Meeting on Feb. 24-28, 1985 and published by the Metallurgical Society of AIME in Light Metals 1985 at pp 1224-1248, a model for cake filtration was presented where the cake and filter media resistance to the flow of molten metal was discussed and quantified. Filtration experiments using reticulated foam filter media with three different alloy melts were described. In this paper, it was pointed out on page 1232 that the rate of filtration decreases with increasing filtration time due to increasing flow resistance offered by the cake.
It would, therefore, be desirable to provide a filter media system which, while taking advantage of cake mode type filtration, would not experience the rapid clogging, as well as filter media degradation, previously experienced with such cake mode filtration.